Conventional powered screwdrivers and impact drivers are known. These drivers receive an insert bit having the desired tip and are used to insert or remove screws. The conventional powered screwdrivers are similar in operation to an electric drill, in that they exert a continuous rotational force onto the bit inserted therein. Typically, the impact drivers function similarly at low torque, but above a predefined torque, instead of a continuous torque, the impact driver assert a series of impacts onto the insert bit. For example, known impact drivers can assert as many as 2800 impacts per minute onto the insert bit being used. In situations requiring high torque, such as inserting a screw into a hard lumber without having drilled a guide hole, impact drivers are able to insert or remove a screw better than conventional powered screwdrivers. Known impact drivers can set a screw so that it takes a torque of, for example, from 30 to 300 Newton-meters ("Nm") to move the screw from the set position.
It is known to austemper insert bits of tool steel to improve resistance to fatigue. Austempering produces bits having a microstructure of bainite. The insert bits are heated to a temperature above 723.degree. C. so that the steel is first austenitized. Then, the steel is cooled to a temperature above the martensite start temperature, for example, around 300.degree. C., and held at that temperature for a desired time to permit the transformation to bainite. No tempering is required. Austempering helps to reduce distortion or cracking during cooling and produces a tool having improved toughness, when compared to tempered martensite at the same Rockwell C hardness ("HRC").
Electro-polishing is a known process wherein a chemicals and electrical current are used to remove surface imperfections. Known uses are in the medical, automotive, food service, and electronics industries. However, electro-polishing of tools, such as impact bits of the present invention is not known to the inventors.
European Patent Office Granted Specification No. 89 104 239 (EPO Application No. 0 336 136) teaches in claim 1 a screwdriver bit with terminal drive region for insertion in a machine chuck, an intermediate section and a power region having a screwdriver tip (7) which has driving ribs (8) arranged uniformly over the periphery for engagement in cross-slotted screws, the apices (8') of which driving ribs (8) project radially beyond the cross-section of the intermediate section (10), characterized in that the intermediate section has a twistable zone, the ratio of diameter to length of the intermediate section (10) having constant cross-section is less than 0.5 and greater than 0.2 and the torsion modulus of the twistable zone T is such that the restoring angle resulting from the resilient recovery power of the intermediate section (10) to reach the torque leading to plastic deformation/break is 3.5 to 8 degrees.